Transmission system



Dc. 17, 1929'. C, w, GREEN ,739,668

TRANSMI SSION SYSTEM Filed Aug; s1, 1926 wur QA EE mf warm/ PatentedDec. 17, 1929 UNITED .srAri-:s PATENT oFFlc-E CHARLES W. GREEN, orMILLRU'RN, 'NEW JERsEY, AssIeNoR 'rofBELL TELEPHONE LABORATORIES,INcoRroRATEn, E NEW YORK, N. Y., A CORPORATION 0E NEW t lrRANsMIss'IoN-SYSTEM Ajpnation mea August' s1, 1926. serial No. 132,670.-

This inventionrelates to transmission systems, and particularly tocarrier current s igi naling systems.

An object of the invention is lto reduce re- -tlection effects such 'asare commonly dueto impedance irregularities in Ielect-ric signalingsystems. A related object. of the invention is to repeat a plurality ofwaves of different. frequencies efficiently yand without substantialreflection.

The 4invention is particularly adapted for Ause in multiplex carriercurrent telephone Whenthe waves in traversing the line meet an impedanceirregmllarity, portions'of the i materially'reduced by means of anattenua-` and telegraph systems, where several conversations or messagesin the form of modulated carrier waves are transmitted inbothvdirections over a common transmission line."

waves are reiiected ck over the line.- Such reflection usually .occursat translating dezvices which are connected to`th e line but glo r `notaccurately match the impedanceof the line. The reflected waves set4upcross-talk adjacent v paralleling lines, thus interfering with thenormal transmission of signals n thereover.

According "togthis invention .reiiection -is tion netwo'r which is :sosituated and proportioned 'with respect 'to the device cansing.reflection that the reliected waves are greatly' attenuated.

' transmission lines, "the waves of higher fre quencles are more greatly.attenuated 1n trav- It is well lmown that systems of this type,particularly those Aemploying 'long designed to have substantially 'the.same highA voltage transformation ratio at all frequencies within thetransmitted band. However, in designing thev repeating coil to havefthedesired 'high voltage transformation ratio over a 'wide band offrequencies, i'thas been' 'found that the .characteristic impedance of"-th'e' repeating coil doesnot match the -im pedance of the incomingyline at 'all the transmitted frequencies. It 'is evident, therefore,thatsreflection will occurf-a-t that partof the frequency scale wherethe impedance of the repeating coil does not accurately matchl theimpedance of the incoming line.. Reflection often 'occurs' at that partof the'l frequency scale where the attenuation of the' equalizer` 1slowest, Aand hence reiiected waves are readily transmi-ttedthrough theequalizer-to the v transmission line. rf According to a feature of thisinvention the impedance of the' repeating coil is made of' the propervalue to tejnlin'ateA the incoming line at that part of the frequencyrange at which the e ualizer'loss'isflowest,'namely,at l

the higher' requencies, so 'that no reflection vwill occur at the higherfrequencies.y Then,

even t h ouh reiiectionoccursat. other fre# tenuated byl traversing theequalizer twice, thus reducing the reiection coetiicient to a minimumfor a xdesired'maximum coil step- -up ratio.

The features and advantages of the invention will be described inconnection with the accompanying drawing, in which:

Eig. 1 is' a .diagrammatic illustration of a twway amplifying repeaterembodying the invention' 5 Fig. 2 shows ciirves illustrating thereduction in the vrei/ection coeiiicient 'obtainable by means oftheinvention; and Figs. .3a and'Bb are circuit diagrams used inconinnctio'n with the. curves ofV F ig. 2 in Idescribing` the invention.

The twoway repeater of Fig. 1 comprises two `one-way repeating channelsRW and RE adapted'to connect transmission line sections' and E. Thisrepeater is of the general type @disclosed `in U. S. Patent No.1,413,357

to P.'A. Baibourn, issued April 18 1922.

The currents incomin at 'the repeater from -line section are amp ifiedin the channel RWand the amplified currents are delivered lto the line.section E. Similarly, the currents incoming from line' section E areamplified in the channelREfandthe'am lified currents are delivered toline section The vcurrents incoming from the line section W are ofhigher frequencies than those incoming from line section Accordingly,the respective repeaterchannels are provided with frequencydiscriminating filters which direct the incoming currents to the properchannels. 'Therepeater channel RW fincludes a high passv inputl filterHPIF, an

amplifying structure WA -and ahigh passv voutput filter HPOF. Therepeater channel RE includes a low pass input filter LPIF, an

amplifyin structure EA and a low pass output filter POF.

The directional filters HPIFand HPOF are designed to transmit currentsyof the ptosuppressvcurrents of higher frequencies.' These-filters may beof the ty e disclosed in A.`.Cam pbell' U. S. Patent No. 1,227,113 toissued May 22, 1917.

Since the attenuation ofthe transmission line sections W and VE is af1mction of frequen'cy, the carrier currents incoming at the repeaterwill vary in magnitude corresponding with the attenuation of the linesection which they have traversed. When these currents Yare repeated`into a succeeding linesecf tion the variation in magnitude at thereceivfing end of the second line section-is proportionally much greaterVthan for the. first line section, and hence equalizers are-usuallyemployed at the repeater to equalize or compen-` sate for the unequalattenuation of the-incominocarrier currents.

the channel RW, an attenuation equalizer WE is connected to thel hi h'pass input filter HPIF to :equalize the ifferences of line attenuationat the upper group of carrier i' frequencies transmitted Vfrom west toeast.

The-.equalizer WE is coupled to the'inputl circuit of the amplifierWA-by means of a repeating coil T. Similarly, an attenuation 4'equalizerEE is-.connected tof the low pass input filter LPIF in the channel RE,thisi;

equalizer being connected to the input circuit of th am lifier EA bymeans of a re coil 1. The equalizers WE and E have transmission losscharacteristics .which are substantially complemental'to thetransmission loss characteristics of -linesections W`K and E,respectively, so that'all the .waves in the output ofeach repeaterchannel will be at? substantially `the sametransmission level.

The ualizers WE and EE may be of the '.-typejdisclosed in-U. S. PatentNo..1,591,073,

issuedV July 6,1926, to D. J. Zobel, having a nst'ant resistancecharacteristic impedance,

ting

edance to terminate the filters PIF, respectively.- y

In order lto obtain thedesired high` gain at all the 'frequencies'assigned to the channel RNV, for example, th .repeating coil Tisdesigned to have the same high voltage transformation ratio at suchfrequencies;v -vSimilar 'conditions apply to the design of the repeatingcoil T1 inthe. channel RE. In one. s ecie instance, a repeating' coilemployed or this purpose was designed to have a voltage step-up .ratioof 1:111. When the "repeating coils are designed to have a high 4voltagetransformation ratio overa wide band of frequencies, it hasl been foundimpossible atthe Sametime to give the coils a characteristicimpedancef'suitahle for terminating the respectivel input filters, thatis, a characteris tic impedance which is constant with frequency.Accordingly, waves of certain frequencles willmeet an impedance irre'ul'arity lin the` repeating coils 'I and T1 an a portion of these waveswillr'be refiected.

According" tothe invention the repeating proper im edan for terminatinthe input filter .to w ich it is`cnnected at at part of the frequencyband at which the ualizeris' ractically transparent,v that is', att ehigher requencies. If, then, the coil impedance-differs from what itshould be at the other frequencies, any reflection introduced by therepeating coil due tol impedance mismatching is reduced bythe relativelyhigh loss `in the equalizer. :This is due to the factthat the Yreiiected wavesfmust traverse. the equalizer' twice, whereas the s'gnaltraverses' the equ'al-v izer but once, so that the reflected waves arehighl attenuated. High resistance elements r an r1 are connected inshunt .to the -primary windings of repeating coils T and T1,

respectively, to-Inakel the impedance of. the

coils correct to a firstapproin'mation.

' Figs. 2, 3 and 3P illustrate how, by'- properly dsigning'the impedanceof a repeater input coi 'the overall reflection coetiicient consideredas a function of frequency reduced to a minimum for' a desired When awave in traversing a #.uit such as that shown-in Fig. 3*meets'a'f-*clan'ge in maximum im edance in the repeating coil T suchthat. 'i

loo ing one,v way the Aimpedance ris `Zland looking the other way theimpedance is Z2,

reflection -of the wave occurs. The coeiiiecient is represented by YZl-Zz R f Z1 The'coeicient of reflection under these conof refiection R`ditions, when no loss element is employed in the circuit, is high, asshown by th curve A of Fig. 2.

Whe1I,- as shovm .in Eig-3", a loss element :,vaaecs tion reducingstandpointA at any frequency. varies as ltwice the loss` through theinserted element at that frequency. Thus, i'

R i with equalizer R l with coil alone varies as'twice the lossV throughthe, equalizer. The coefficient' of reflection when the equalizer `E isemployed between the line and the V*repeating coil 'I is materiallyreduced, as shown by the curve B of Fig. 2. In an actual installation a3% fold improvement in reflection coefficient has been obtained by meansof this invention with the same repeatergain as has been obtainedheretofore. Looking at the advantage of the invention from anotherviewpoint, vif the same reflection coeicient heretofore obtained is notobjectionable in certain installations, the gain can be increasedmaterially' by means of the invention, without changing the reflectioncoefficient,

While the invention has been described as` 'tions and in other types ofsystems.

What is claimed is: 1. In a system for'transmitting waves of differentfrequencies, a transmission line,

means associated with said line causing waves of certain selectedfrequencies to be reflected, and a loss element-between said line andsaid .1 means to attenuate'all of said reflected waves,

said loss' element having its .highest transmission loss-characteristicsat the frequencies of said reflected waves and a lowerloss at the other frequencies transmitted.'

2. In a system for 'transmittingwaves of different frequencies, -atransmissionv line, means comprising' a. transformer associated withsaid line causing. waves of vcertain selected frequencies to bereflected, said transformer having a high voltage transformation A,ratio substantiallyconstant.over the range of frequencies transmittedand a loss element in means-'associated with said line causing waves vcf certain selected frequencies to be reflected,

anda losselement associated with said line and-arranged to produce agreater loss for all the reflected waves than for the Waves .which aretransmitted without reection.

4. In a system for transmitting waves of 4different frequencies, a lineover which said waves ofdiiferent frequencies aresreceive'd,

means for 'reducing reflection effects due to..

limpedance .irregularities including a circuit comprising an electricalnetwork upon which said waves areimpressed, said circuit having atransmission floss characteristic which is high at'certain frequenciesand lo'w at other frequencies, and a translating device comprising atransformer having a high voltage transformation ratio substantiallyconstant over the range of 'frequencies transmitted connected to receivethe waves from said circuit and havingv the same impedance as said.circuit at frequencies at -whichsaid circuit has a low transmission`loss and a different impedancev at the other frequencies transmitted;

5. In a system -for transmitting waves of different frequencies, a line.over which said waves of different vfrequencies are received, means forreducing reflection effects due to impedance irregularities including acircuit comprising an electrical network upon which said waves areimpressed, said circuit having a transmission loss characteristic. which.is high at' certain .frequencies and low at other frequencies, andatranslating device connected to receive the waves from said circuit andhaving ari impedance which varies throughout thera'ige of transmittedfrequencies but accurately matches the impedance of 'said circuit atthe, frequencies at which said circuit has a low transmission loss.

6. In a system fortransmitting waves o different frequencies,- a' lineover which said waves ofv dierent frequencies are received, means forreducing reflection effects due to impedance irregularities including acircuit comprising` an' electrical network -upon which said Waves areimpressed, said circuit having a'transinission loss characteristic whichis high atv certain frequencies and low at other frequencies, and meansconnected to receive the waves from said circuit comprising a re-.p'eating coil having a high transmission efiiciency at all of saidfrequencies and having approximately the same impedance as said circuitat-vfrequencies at which said circuit has a low 'transmission loss and adifferent ido llU

impedance "at the other frequencies transmitted. L

7. In a system for transmitting waves of different frequencies, a lineover which said waves of dierent frequencies are received, means forreducing reflection effects due to impedance irregularities including acircuit comprising an electrical network upon which` impdanceof'saidcircuit' atthe frequencies at -y/,h'ichfsai-id loss..' 8. In a wavetransmission system employing waves of different vfrequencies, aline thetransmission loss of which difers'for dif'- erent frequencies,- anattenuation equalizer associated .therewith andv having a trans"- emission loss characteristic' substantially com-y cies.

plementaltothat 4of the line throughoutthe rangeqof frequencfusItransmitted, and a translatingdevice conn ted; to said equalizer andhaving an impedance which matches the imi pedance. of said lineJ andequalizer only in the upper region of the transmittedfrequentransmission system employ` ing' Waves of different frequencies,a line the transmission loss of which differs for different frequencies,an attenuatlon equallzer associated therewith andl having a transmissionloss characteristic substantially complemental to that of the linethroughout the range 'of frequencies transmitted, an amplifier, and atransformer connecting said attenuation equalizer and amplifier. andhaving an impedance which .varies throughout 4the range of frequenciestransmitted but' matches the i impedance of said-,equalizer atfrequencies r transmission loss.

at which said attenuation equalizer has a low 10. In ,a Wavetransmission system, a line l over, which signaling waves aretransmitted,

duced by said element a translating device from which waves of certainselected frequencies are reflected, and

'a loss element arranged to be traversed by the reflected Waves agreater' number of .times Wavesv the loss introcing substantiallyproportional to v'the amount of reflection` at the differentfrequencies.

11. Ina systemfor transmitting waves of than by said signaling differentfrequencies, a transmission line, a`

repeater. associated with said line and comprising a frequencydiscriminating filter, an ampli erand a repeating coil connecting saidampliierto said-filter, and,a loss element Abetween Asaid filter andsaid repeating coil having a transmission loss characteristic which ishigh at certain frequencies and low at other frequencies, said repeatingcoil haw ing an impedance which varies throughout the range offrequencies transmittedby said ilterrbut matches the impedance of saidfilter at frequencies at which said loss element has aV low transmissionloss.

12.v The method of'reducing cross-talk' in signaling' circuits whichcomprises reducing the reflection of waves inthe upper region of thetransmitted frequencies to a greater ex `tent than' in the'loy'verregion, attenu'ating ,circuiti has' ai low transmission frequencies to aygreater extent'than the upper region.

13. In al system for transmitting waves of Y.

pedance matched to that of the incoming'cir- Y cuit comprising the line4and equalizerat the frequenciesl at which *said equalizer has thelowestv attenua-tion andmismatched at other frequencies, said repeatingcoil having its voltage ratio substantially constant lover the lrangeofafrequex'icies transmitted and as high as is consistent' with` th@permissible extent of'mismatching of the impedance ofsaid'reof theincoming circuit, as4 determined by a substantially constantandnegligibly small amount of reiiectionrappearing-on the line side Aofsaid equalizer throughout the transmitted range. y

14. rIAhe method of reducing reiection eftem for transmitting waves ofdifferent fr quencies' which comprises 'attenuating the; waves inan'inverse relation with respect to frequency, reflecting waves in thelower` region ofthe transmittedXfrequenciesto a greater degree thantheother waves transmitted, and attenuating certainv reflected Vfectsdue to impedance irregularities in a sys'n waves after reflection in aninverse relation s with respect to frequency.A l 1 l15. The meth d .of'securinga high voltage .transformation ratio over a Wide band offrequencies Without substantial. reflection effects in a repeater for asystem transmitting-waves of different frequencies which comprisesattenuating the waves aninverse` ratio 'to the line attenuation, makingthel reflection of waves a minimum. in the region of the transmittedfrequencies at Whichfthe line at? ros tenuation is high and attenuatingthe waves of other frequencies after reflectionin an inverse ratio tosaid line attenuation.l Y, n In witness whereof, 'I hereunto subscribereflected waves in said circuits a greater num ber oftimes than ltheeffective signaling currents therein and attenuating 'the reflectedlwaves in the lower. region of the transmitted` 85 peating coil'withrespect to the impedancel lit my name this 30th day of August A. D.,1926'.

